The present invention relates to a downshift control in a motor vehicle with an automatic transmission.
U.S. Pat. No. 4,730,521 discloses an automatic transmission for a motor vehicle. This known automatic transmission comprises a plurality of planetary gear sets and it is shiftable from a current gear ratio to a new gear ratio by changing relationship in connection among constituent elements of the plurality of planetary gear sets by engaging or releasing at least one of a plurality of frictional devices, i.e., brakes and clutches.
In the known automatic transmission, a line pressure is used as a servo activating hydraulic fluid pressure for activating the frictional devices. The line pressure is variable in response to engine load (i.e., throttle opening degree) such that the line pressure is increased during operation with high engine load to prevent occurrence of a slip in the frictional devices.
Upon making a shift from a current gear ratio to a new gear ration, a fluid operated frictional device associated with the current gear ratio which is engaged with the line pressure is released by discharging hydraulic fluid from the frictional device, and subsequently hydraulic fluid is supplied to another fluid operated frictional device associated with the new gear ratio.
JP 62-88856 A teaches that magnitude of a line pressure is varied upon making a shift.
Referring to FIG. 9, the known automatic transmission disclosed in U.S. Pat. No. 4,730,521 is further described. FIG. 9 is a timing diagram during a 3-2 downshift when hydraulic fluid is discharged from a fluid operated frictional device, i.e., a high clutch, associated with the third gear ratio immediately after a command for the 3-2 downshift (the instant t.sub.0 in FIG. 9) and subsequently hydraulic fluid is released from a release side of another frictional device, i.e., a second brake, associated with the second gear ratio. In this timing diagram, servo activating hydraulic fluid pressure within the high clutch decreases along a curve L, and engine revolution speed increases along a curve E. The hydraulic fluid pressure within the high clutch stays as high as the level of line pressure as illustrated by a broken line P.sub.L for the duration of time t.sub.1 after the instant t.sub.0. Thus, the high clutch remains engaged to keep applying substantial load on the engine, so that an increase in engine revolution speed immediately after the instant t.sub.0 is slow. In other words, the increase in engine revolution speed is slow when the duration of time t.sub.1 is long.
FIG. 10 shows engine revolution speed vs., vehicle speed characteristics with the first, second and third gear ratios as parameters of the known automatic transmission in question. As readily understood from FIG. 10, an increase in engine revolution speed during 3--2 downshift at a vehicle speed V.sub.1 is represented by a double headed arrow E.sub.1, and an increase in engine revolution speed during 3-2 downshift at a vehicle speed V.sub.2 is represented by a double headed arrow E.sub.2. This indicates that an increase in engine revolution speed during 3-2 downshift becomes great as the vehicle speed increases.
Thus, it is required to induce a quick rise in engine revolution speed at high vehicle speed. However, the presence of residual fluid pressure during the duration of time t.sub.1 causes the rise in engine revolution speed to occur at a delayed instant during 3-2 downshift at a high vehicle speed. This results in an increased duration of time required from the occurrence of the command (at the instant t.sub.1) to the completion of the downshift, i.e., at the instant when the engine revolution speed reaches a new target value for the second gear ratio, inducing a shift lag.
In order to shorten the duration of time t.sub.1, it has been proposed in JP 62-88856 A that the line pressure is decreased temporarily during downshift. This proposal is effective for downshift at a high vehicle speed. However, if this proposal is put into effect during the same downshift at a low vehicle speed, engine racing is induced since as shown in FIG. 10, the increase in engine revolution speed required at low vehicle speed is not great.
An object of the present invention is to improve a downshift control such that a downshift at a low vehicle speed is not accompanied with engine racing, while the same downshift at a high vehicle speed is made without any shift lag.